Windmill Farms
Windmill farms, or wind power plants, are normally designed to generate power at low voltage or medium voltage. When the generator is generating at low voltage (typically <1000 V) a step-up transformer on board the windmill is used to increase the generated voltage to typically 12-36 kV to reduce the cable size (and cost). A medium voltage generator, typically 12 kV, is an alternative which eliminates the need for a step-up transformer onboard the windmill.
The power from the windmill farm is normally collected in a windmill transformer substation, either underwater, or on a supporting structure above the water level (or on land).
The windmill substation can collect the power from a cluster of windmills and further step up the voltage to reduce cable cost and allow transfer of more power.
The high voltage cable from a cluster of wind power generators is normally collected on a central platform, which collects the power from a number of wind power generator clusters. This platform can transport the power to land via High Voltage DC or AC cables.
Such multi-ended power networks are complex constructions and require a number of protection devices.
EP2282054 (D5) describes a wind power plant, wherein a plurality of wind power generators, each including a transformer (40 in D5), are grouped and connected into a common feeding cable (44 in D5), which feeding cable is further interconnected with another similar feeding cable to a substation transformer (12 in D5). The substation (48 in D5) in turn provides a grid connection (34 in D5). For protection against faults occurring in the system of feeding cables, circuit breakers are arranged in each end of the cable system, i.e. one in each end of the two common feeding cables (44) and one circuit breaker (62) at the connection between the cable system and the substation transformer (12) (§45 in D5). Arranging protection equipment in this way and having a circuit breaker in both ends of each cable requires a lot of equipment if such a protection system is arranged at a wind power plant having many power generators. This kind of protection adds costs when the number of power generators, and cables from the power generators are increased in order to scale up the wind power plant.
Differential Protection
The invention provides differential protection of a wind power plant using protection equipment similar to the technology used for protecting transmission lines in power grids. Such equipment is not discussed in detail in document D5. Therefore a short survey of differential protection methods and systems used, from other fields than wind power plant protection, will be provided in the following.
Differential protection is a well known method for protecting transmission lines. For example, UK patent application GB 523,603 describes differential protection for AC systems using differential relaying for protecting power lines, power transformers, A.C. generators and station bus systems, wherein each phase of the current entering the system is compared with the current leaving the system (see D1 page 1, lines 10-14, 32-46). The ratios of the power transformers of the system is compensated for, by choosing the measuring current transformers to have corresponding ratios, so that the currents from the current transformers should be a total of zero, i.e. ingoing currents equal to outgoing currents of a protected zone of the system, when there is no fault in the protected zone. When a fault occurs in the protected zone (1 in FIG. 1), a differential current is created, which current causes a protective relay to operate and trip circuit breakers and disconnect the faulted equipment (see D1, page 1, lines 46-77). Moreover, the protective circuit of D1 (see for example FIG. 1 of D1) includes an operative winding 13 that is trigged by a differential current, but also a restraining coil 12 that is trigged by an inrush current, wherein the restraining coil 12 compensates for inrush magnetizing currents of transformers in the protected zone (see D1 page 2, lines 73-93). In this way the protective circuit of D1 prevents false operation of the differential protective arrangement by a restraining effect, which is adapted to react upon a harmonic of the secondary current of the measuring current transformer, so that the relay operates only when a fault exists in the protected zone (1) (see D1 page 3, line 28-47). The protected zone (1 of D1) comprises for example a combination of two or more of a power transformer, machine, station bus and power line and in the example illustrated in FIG. 1 of D1 three circuit breakers 5 connect three power circuits 2, 3, 4 to the zone 1 (page 3, lines 106-130). The current to and from these circuits are monitored by current transformers 8 of the protective circuit.
A magnetizing inrush current affects the differential current, which is illustrated in FIG. 4 in D1, and the differential current shows a high second harmonic (page 4, lines 43-54). The protective relay circuit is designed to restrain on this second harmonic (page 4, lines 67-79) and also on currents from a saturated measuring current transformer, for example by means of a restraining coil 12 (page 4, lines 114-120). Thus, D1 describes a system for differential protection, wherein the protective circuit compensates for inrush currents of transformers, so that when a switch is closed to apply voltage to a transformer winding, the protective relay is restrained from tripping the circuit breaker.
FIG. 10 of D1 illustrates protection of a power transmission line wherein a pilot conductor (one for each phase 48, 49, 50) is used for transferring measuring currents from the current transformers 8 that measure the currents entering and leaving the transmission line (45-47).
An alternative to use pilot conductors is described in U.S. Pat. No. 3,223,889 (D2), wherein a protective circuit, for protecting a power circuit, uses for example radio (see D2, FIG. 2 and column 2, line 30-32) for transmission to a trip control means. FIG. 6 of D2 illustrates employing a radio link for transferring control signals including a second harmonic of the power circuit (see D2 column 2, line 45-54).
Differential protection have since been developed further becoming more and more sophisticated and intelligent. WO2007/051322 (D3) describes an arrangement for protecting an electrical power system comprising a plurality of protective relays provided with a respective phasor measurement facility for measuring synchronized current values at different locations and including a respective data communication module for communicating such current values with each other via a communication link (see abstract of D3). Such systems use Phasor Measurement Units that provide time-stamped measurements, so called phasors, of currents, voltages and loads, being synchronized and time-stamped by means of Global Positioning Satellites (e.g. GPS) (see D3 page 3 line 1-12). The communication link may include an optic fiber link. D3 also describes line differential protection of a multiterminal system, illustrated by five terminals in a protected zone (illustrated by dashed line in FIG. 2), which protected zone is defined and protected by five respective relays (2), one relay for each terminal end (page 8, line 23-29).
US2011/0063767 (D4) describes line current differential protection of a line, or a multi-terminal line system, which protection includes compensation of charging currents. Each end, or node, of the system measures the current and compensates for the charging current of its cable, before (see abstract, FIG. 18, and §67 of D4) transmitting a current value. Thus, each node sends a compensated current value calculated from the measured current and a subtracted charging current to the other nodes. Each node also calculates (§68) an overall differential current from its own transmitted current value and all received current values of the other nodes. The sum of the charging currents used in the nodes should equal the total charging current of the cable system (§69).
The paper “Practical experience from multiterminal line differential protection installations” by Z. Gajic et al (document D6) from the conference Relay protection and Substation Automation of Modern Power Systems (Cheboksary, Sep. 9-13, 2007) describes experience from protection tests. The document D6 describes two installations, a first installation protecting a five terminal line, and a second installation protecting a three end line. In each end of the lines, a circuit breaker and a current and voltage measuring device is arranged. The installations use a line differential protection unit for measuring the current and voltage, and tripping the circuit breaker, which protection equipment unit is of type RED 670, provided by ABB. The protected zones do not include any transformers. However, such equipment like RED 670 can handle transformers and charging currents within a protected zone. For example, charging currents can be handled by an RED 670 by reducing its sensitivity a short time period during charging of a cable, so that the charging current is not interpreted as a fault current. The invention proposes a system for protection using similar protection equipment modified for controlling more complex systems, like multi-terminal systems for transfer of power from several generators.
U.S. Pat. No. 6,456,947 discloses a method of detecting faults on a power transmission line system including simultaneously measuring phase current samples at each phase of each transmission terminal; calculating real and imaginary phaselets comprising partial sums of the phase current samples; for each phaselet, calculating a respective partial sum of squares of each phase current sample; calculating the sums of the real and imaginary phaselets over a variable size sliding sample window; calculating real and imaginary phasor components from the phaselets and a sum of the partial sums of the squares over the sample window; using the sums of the real and imaginary phaselets, the real and imaginary phasor components, and the sum of the partial sums of the squares to calculate a sum of squares of errors between the phase current samples and a fitted sine wave representative of the real and imaginary phasor components; using the sum of squares of errors to calculate a variance matrix defining an elliptical uncertainty region; determining whether a disturbance has occurred, and, if so, re-initializing the sample window; and determining whether a sum of current phasors from each terminal for a respective phase falls outside of the elliptical uncertainty region for the respective phase.
US 2011/0063769 discloses current differential protection for a multi-terminal power apparatus, such as a power transmission line. Currents measured at each of the multiple terminals are used to calculate a differential current and a restraining current, which are then converted into a first equivalent current and a second equivalent current of an equivalent two-terminal power apparatus. In the equivalent two-terminal power apparatus, a differential current derived from the first and second equivalent currents is substantially equal to the differential current of the original multi-terminal power apparatus. Similarly, a restraining current derived from the first and second equivalent currents is substantially equal to the restraining current of the original multi-terminal power apparatus. The first and second equivalent currents may be used in an alpha plane analysis to determine whether or not to trip the multi-terminal power apparatus.